The present invention relates to a metal separator for fuel cells, formed with a metal plate and provided between cells accumulated, and a fabricating method thereof. In the metal separator, the metal plate is formed like trapezoid irregularities to separate channels for a fuel gas from ones for an oxidant gas. The present invention especially relates to a metal separator for fuel cells with the shape of high accuracy and a fabricating method thereof, in which the processing characteristic of the metal plate is improved before molding one.
A proton-exchange membrane fuel cell is a fuel cell which uses the film of solid polymer electrolyte which passes an ion. A structural body that an anode electrode and a cathode electrode are placed in the inside and outside of the solid polymer electrolyte is called an electrolyte-electrode complex. Electrical energy is generated by the chemical reaction energy of a fuel gas and an oxidant gas by supplying the fuel gas to the anode electrode side of the electrolyte-electrode complex, and supplying the oxidant gas to the cathode electrode side. The typical one as the fuel gas is hydrogen, and atmosphere is used as an oxidant gas. Usually, fuel cells are piled up in plural layers to obtain a high electric power because a voltage which can be taken out by this electrochemical reaction is low, about 1V. One layer of the fuel cell is called a fuel cell, and plural layers of fuel cells piled up are called a fuel cell stack.
When the fuel cells are piled up, a separator is used to separate oxygen of an cathode electrode from hydrogen of an anode electrode of the adjacent cell. The separator only separates neither oxygen nor hydrogen but also efficiently supplies hydrogen or oxygen and drains the produced water through a minute pit of about 1-3 mm in depth and about 0.5 mm in pit space formed on a surface. Moreover, the separator plays role in collecting the electron from the anode electrode and supplying to the cathode electrode of the cell which adjoins.
As for the separator which plays role like the above-mentioned, Permeability of hydrogen be low, the formability which can process a minute pit to the surface, low contact resistance to collect electrons, high electric conductivity, and high corrosion resistance are requested in order to separate hydrogen and oxygen.
There is problems in the toughness and the processing characteristic though the graphite has been used as a material which meets these requirements demanded as a separator.
The separator which uses a metal is also used. The advantages of a metal separator are in that the productivity can be remarkably improved by using press molding to reduce the cost, and the fuel cell can be downsized because it is possible to thin due to high toughness. On the other hand, the biggest problem for the use of the metallic material is deterioration in cell characteristics due to corrosion. To cope with this problem, cheap materials such as iron, stainless steel, aluminum, and copper are used as a base material, and precious metals such as Au, Pt, etc. which satisfies both corrosion resistance and high electrical conductivity are plated on the base material. Or, Ti that the corrosion resistance is high is bonded on the base material by rolling etc., and the coating processing is carried out to prevent contact resistance.
With regard to a forming method of the pit of a metal separator, press molding is chiefly used. In this method, the separator is formed in one or several processes. Because one process completes within several seconds, productivity is high. This forming method has merits and demerits. There is a problem with a low degree of freedom of shape compared with a separator made of graphite. However, there is a merit that a compact cell stack can be made.
JP 2005-190968 A discloses a method of controlling contact resistance, in which a contact side of a electrolyte-electrode complex is made smooth by forming pit form once and forming the pit form again flatly after the top portion is curved. However, this method is difficult for the material that the deformation degree of irregularities fabrication is large, and the expansion is low.
JP 2007-149478 A discloses a method of controlling contact resistance, in which the decrease in board thickness in the corner portion is compensated by using a metal mold having convex corners in the fabrication of the separator, and the contact side of an electrolyte-electrode complex is made smooth. The processing characteristic of the material can be somewhat improved even by this.
JP 2005-190968 A and JP 2007-149478 A disclose a method in which the conventional stretch molding method is partially improved. In a material having bad processing characteristic, tensile elongation reaches its limit, thus cracks are apt to occur, because in these methods tensile bending stress is applied to outside corner 112 where the stretch molding is carried out as shown in FIG. 11.
JP 2002-75401 A discloses a method in which concave sides 122 of irregularities of a separator are pressed against with rounding tool 123 to thin the wall thickness of a point of a rounding portion, and thereby convex sides 124 on the other side are made smooth by just that much (Refer to FIG. 12). However, the tensile stress is occurred as well as the stretch molding, and the possibility to crack is high for a material having a bad processing characteristic in the first deformation process. In this method, the processing characteristic is not improved effectively though the compressive force is applied to decrease wall thickness after familiarizing with a mold.